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TOPIC: VLSM Table

VLSM Table 10 years 1 month ago #16017

  • Nayyar
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Hello there,
I am studying Sybex CCNA Study Guide Fifth Edition wrote by Todd Lamle.
I just have question in regards to the Valid subnets for Class C address.
Sybex book says for 192 mask it gives you 2 subnets . I think it should be 4 subnets.

Can anyone clarify this?

Thanks,

Nayyar
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Re: VLSM Table 10 years 1 month ago #16024

  • reaper
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It depends on if you use ip subnet-zero or not. If you don't use it you will only have two usable subnets because the all zero subnet (00) and the all 1 subnet (11) are not allowed.

If you have configured ip subnet-zero you may use these subnets and then you have 4 usable subnets.

If it doesn't say to use ip subnet-zero don't count the first and last subnet. Even though this command has been default for a long time in the real world it's just one of those things Cisco likes to ask about on exams.
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Reply to reaper 10 years 1 month ago #16038

  • Nayyar
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Hello there,
I just read your comment. So if in CCNA exam it don't specifically mentioned about subnet zero, then it means 255.255.255.192 have 2 usable subnets.
For example, with subnet zero (it will give us 4 usable subnets) and the table will be as follows:

0 64 128 192
1 65 129 193
62 126 190 254
63 127 191 255

and without subnet zero (it will be only 2 usable subnets) and the table will be as follows:

64 128
65 129
126 190
127 191

Am I right?

Thanks,

Nayyar
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Re: Reply to reaper 10 years 1 month ago #16039

  • d_jabsd
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Hello there,
I just read your comment. So if in CCNA exam it don't specifically mentioned about subnet zero, then it means 255.255.255.192 have 2 usable subnets.
For example, with subnet zero (it will give us 4 usable subnets) and the table will be as follows:

0 64 128 192
1 65 129 193
62 126 190 254
63 127 191 255

and without subnet zero (it will be only 2 usable subnets) and the table will be as follows:

64 128
65 129
126 190
127 191

Am I right?

Thanks,

Nayyar

Yes, but i'm not sure what your tables mean since they don't have labels.

Unless they say subnet zero is enabled, it is usually safe to assume it is not, so you would be left with 2 usable subnets-

x.x.x.64/26 and x.x.x.128/26.
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VLSM problem 10 years 1 month ago #16118

  • kwyap
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Hi,

I'm studying BSCI, i have a question regarding VLSM. This is my scenario;

With 192.168.10.0/24, I have 2 sites, each site has 2 different building, each building has 15 hosts.

My original plan,
192.168.10.0/27 for 15 hosts

Is it possible to allocate 192.168.10.28/30 for my core layer subnet? Will it over lap with the first range for my first building which is 192.168.10.0/27 ?

Any suggestion would be much appreciate.

Regards,
kwyap
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Re: VLSM Table 10 years 1 month ago #16247

  • reaper
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Yes, it will overlap so it is not possible. If you have a netmask of /27 it means that you have 255.255.255.224, if you subtract 224 from 256 you get a blocksize of 32.
This means that the first subnet is 192.168.10.0 (ip subnet-zero) and the next one is 192.168.10.32. First host in first net is 192.168.10.1 and last host 10.30 and broadcast 10.31.
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